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Controlled Aqueous Atom Transfer Radical Polymerization with Electrochemical Generation of the Active Catalyst.

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Angewandte
Chemie
DOI: 10.1002/ange.201105317
Atom Transfer Radical Polymerization
Controlled Aqueous Atom Transfer Radical Polymerization with
Electrochemical Generation of the Active Catalyst**
Nicola Bortolamei, Abdirisak A. Isse, Andrew J. D. Magenau, Armando Gennaro,* and
Krzysztof Matyjaszewski*
Controlled/living radical polymerizations (C/LRPs) in aqueous media are attractive from both economic and environmental points of view. Aqueous media can be used for the
synthesis of a vast array of hydrophilic and hydrophobic
polymers, through homogenous and heterogeneous (e.g.
suspension and (mini)emulsion) polymerization systems,
respectively.[1] Moreover, efficient C/LRPs conducted in
aqueous saline buffers can be crucial for the preparation of
polymer–biomolecule conjugates under biologically relevant
conditions.[2]
Atom transfer radical polymerization (ATRP) is one of
the most commonly employed C/LRP techniques, enabling
the synthesis of polymers with predetermined molecular
weights, narrow molecular weight distributions, and specific
compositions and architectures. ATRP is often catalyzed by a
CuII/CuI system in which the success of this process relies on a
rapid and reversible activation/deactivation step. In this
dynamic equilibrated system, activators (CuIL+) react with
dormant macromolecular species (RX) to produce propagating radicals (RC) and deactivators (X-CuIIL+; Scheme 1, region
delimited by the dashed line). In nonaqueous solvents the
equilibrium constant, KATRP = kact/kdeact, is usually small
(< 10 4)[3] resulting in a dramatic decrease of [RC] which
consequently suppresses bimolecular termination reactions.
In contrast to the success of ATRP in organic solvents,
aqueous ATRP has been found to suffer from some limitations, specifically with regard to achieving polymerization
control and the targeted degree of polymerization (DP).[4]
These observed limitations in aqueous ATRP may result from
three main phenomena. First, aqueous ATRP has a relatively
large KATRP providing a high [RC] and usually fast polymerizations.[5] Second, the halidophilicity (KX) of CuIIL2+, that is,
[*] Dr. N. Bortolamei, Dr. A. A. Isse, Prof. A. Gennaro
Dipartimento di Scienze Chimiche, Universit di Padova
via Marzolo 1, 35131 Padova (Italy)
E-mail: armando.gennaro@unipd.it
Homepage: http://www.chimica.unipd.it/electrochem/
Dr. A. J. D. Magenau, Prof. K. Matyjaszewski
Department of Chemistry, Carnegie Mellon University
4400 Fifth Avenue, Pittsburgh, PA 15213 (USA)
E-mail: km3b@andrew.cmu.edu
Homepage: http://www.cmu.edu/maty/
[**] We thank the following organizations for financial support:
Fondazione Ing. Aldo Gini (N.B.), University of Padova (grant
CPDA083370 to N.B., A.A.I., A.G.), U.S. National Science Foundation (grant CHE 10-26060 to A.J.D.M., K.M.), and CRP Consortium
at Carnegie Mellon University (A.J.D.M., K.M.)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105317.
Angew. Chem. 2011, 123, 11593 –11596
Scheme 1. Mechanism of conventional (delimited by dashed line) and
aqueous electrochemical ATRP.
association of X to CuIIL2+, is small therefore diminishing the
concentration of deactivator X-CuIIL+.[6a] Third, CuIL+ may
be unstable in water and may undergo disproportionation.
Developing a successful aqueous ATRP requires consideration of all the previously mentioned issues.[6a] In fact,
improved polymerization control was found by using a high
[X ], which helps to suppress deactivator dissociation.[6]
Further development of the process has been recently
achieved through AGET (activators generated by electron
transfer) ATRP.[7] Although this process provides good results
in terms of both control and DP, the correct [CuII]/[reducing
agent] ratio and appropriate reducing agent are critical for
success.[7b] The ideal process should have a constant and high
CuII/CuI ratio, which is difficult to achieve over the whole
polymerization by addition of a single reducing agent.
Herein we describe an electrochemical ATRP method
(eATRP), aimed to fulfill these criteria. The overall mechanism of eATRP is depicted in Scheme 1. Initially, the reaction
mixture contains solvent, monomer, initiator, and CuIIL2+ (or
CuIIL2+ + X-CuIIL+). Under these conditions, CuIL+ activator
is absent in solution, and hence, no polymerization occurs.
The onset of polymerization begins only when a sufficient
potential (Eapp) is applied to the cathode so that reduction of
CuIIL2+ to CuIL+ occurs at the working electrode. The
magnitude of Eapp can be appropriately chosen to achieve a
continuous (re)generation of a small quantity of CuIL+ and
consequently dictate the [RC]. A living polymerization process
is ensured by the combination of a low [RC] and high
[CuIIL2+]/[CuIL+] ratio. Furthermore, the polymerization
rate and degree of control can be tuned by adjusting the
Eapp. The first example of ATRP under electrochemical
generation of activator has been recently reported for the
successful polymerization of methyl acrylate in acetonitrile.[8]
Herein, we describe eATRP of oligo(ethylene glycol)
methyl ether methacrylate (OEOMA475) in water. As a
catalyst system, CuII/I TPMA (TPMA = tris(2-pyridylmethyl)amine) was selected, which is one of the most active
2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Zuschriften
complexes used in ATRP.[9] Cyclic voltammetry (CV) of this
CuIIL2+ complex exhibits a reversible peak couple at Eo’ =
0.245 V versus the standard calomel electrode (SCE)
(Figure 1). Addition of a large excess of Br to the solution
had no significant effect on the CV response, indicating that
the KX of CuIIL2+ is small (see Figure S1 in the Supporting
Information). The full reversibility of this response at low
scan rates also indicated that CuIL+ is quite stable in H2O with
a lifetime of at least a few seconds (duration of CV). In fact,
we estimated a KD value of 6.8 10 3 for the disproportionation of CuIL+ (see the Supporting Information).
Figure 1. Cyclic voltammograms of 1 mm CuIIL2+ in H2O/OEOMA475
(9:1 v/v) + 0.1 m Et4NBF4 recorded at v = 0.1 Vs 1 in the absence
(a) and presence (c) of 1 mm HEBriB; the three dots on the CV
trace correspond to the Eapp values used in the polymerization experiments.
The CV response of CuIIL2+ drastically changed when an
equimolar amount of initiator (2-hydroxyethyl 2-bromoisobutyrate, HEBriB) was added; the cathodic peak approximately doubled in height while the anodic one decreased,
clearly indicating that CuIL+ rapidly reacted with HEBriB.
On the basis of thermodynamic data available in the
literature, we estimated a KATRP value of 1.5 10 1 for this
system (see the Supporting Information), which is 4 orders of
magnitude higher than that measured for an analogous system
in CH3CN.[3a] An estimate of the activation rate constant
based on voltammetric analysis of the system (i.e. CuIIL2+ +
HEBriB) at different concentration ratios (see Figure S3 in
the Supporting Information) and scan rates provided a very
large kact 2.5 106 m 1 s 1.
These voltammetric analyses have shown that the system
under investigation has all the characteristics (low KX, high
KATRP, and rapid activation) expected to result in an uncontrolled polymerization. Therefore, this system represents an
ideal candidate to test the ability of the proposed electrochemical method to control aqueous ATRP. Electrogeneration of the active catalyst was carried out under potentiostatic
conditions starting from the catalytic system CuIIL2+/HEBriB
1:1 in H2O + 10 % OEOMA475 (molecular weight, MW =
475). The effect of Eapp on the degree of polymerization
control was first investigated. Three Eapp values around Eo’
(see Figure 1) were applied and the results are summarized in
Table 1 (entries 1–3).
The driving force of the electrochemical process is given
by DGo = F(Eapp Eo’) and the [CuIIL2+]/[CuIL+] ratio at the
electrode surface is closely related to that dictated by the
Nernst equation. At the beginning of electrolysis there is only
CuIIL2+ present in solution, so the current must decay as CuII
is converted to CuI, thereafter approaching zero as the
[CuIIL2+]/[CuIL+] ratio approaches the value required by Eapp.
However, CuIL+ also participates in a reversible reaction with
alkyl halides (i.e. dormant species), which represents a
perturbation to the equilibrium concentrations imposed by
Eapp. Therefore, whether a constant [CuIIL2+]/[CuIL+] ratio
can be imposed in the bulk solution depends on the mutual
rates of electrogeneration and disappearance of CuIL+ and
therefore will depend on Eapp. Actually, the current decreases
to a small constant value rather than to zero (see Figure S4 in
the Supporting Information) because of the dynamic ATRP
equilibrium and the termination reactions.
At Eapp = 0.55 V, which is ! Eo’, the electrode process is
under diffusion control and CuIIL2+ is almost quantitatively
converted to CuIL+ in a relatively short time; this behavior is
exemplified by the rapidly decreasing current to a minimal
value (see Figure S4 in the Supporting Information). The
overall rate of polymerization was rather high, and 79 %
monomer conversion was achieved in less than 30 min
(Figure 2 a), however, at the expense of polymerization
control. The plot of ln([monomer]/[monomer]0) versus time
deviated significantly from linearity, while the distributions of
MW, expressed as Mw/Mn (where Mw and Mn are the weightaverage and number-average molecular weights, respectively)
were broad (Figure 2 b). These features are typical of an
Table 1: Electrochemical aqueous ATRP of OEOMA475 at 25 8C.
Entry
1
2
3
4
5
6
7
8
9
Monomer
[% v/v]
10
10
10
10
10
5.0
2.5
1.0
5.0
[monomer]/[RX]/[CuIIL]
Electrolyte[c]
200/1/1[a]
200/1/1[a]
200/1/1[a]
200/1/1[a]
200/1/1[a]
1000/1/1[b]
500/1/1[b]
200/1/1[b]
1000/1/1[b]
Et4NBF4
Et4NBF4
Et4NBF4
Et4NBr
PBS buffer[d]
Et4NBr
Et4NBr
Et4NBr
PBS buffer[d]
Eapp
[V vs. SCE]
0.550
0.310
0.210
0.210
0.275
0.210
0.210
0.210
0.275
t [h]
Q [C]
Conv. [%][e]
Mn,theor
[10 3]
Mn,app
[10 3][f ]
Mw/Mn
0.5
1.8
3.0
2.5
3.0
2.0
2.0
2.5
2.0
3.05
4.34
2.65
2.32
1.94
0.175
0.210
0.267
0.179
79
88
98
99
98
84
72
77
79
75.1
83.2
93.1
94.0
93.1
399
171
73.2
375
71.6
74.8
54.1
56.8
58.2
202
106
61.3
177
1.58
1.53
1.22
1.16
1.15
1.25
1.19
1.35
1.20
[a] [CuIIL2+] = 1 mm. [b] [CuIIL2+] = 0.1 mm. [c] 0.1 m. [d] 0.137 m NaCl + 2.7 mm KCl + 11.9 mm (Na2(HPO4) + KH2PO4) in H2O; pH 7.4.
[e] Determined by 1H NMR analysis. [f] Determined by GPC with linear PMMA calibration.[10]
11594
www.angewandte.de
2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2011, 123, 11593 –11596
Angewandte
Chemie
Figure 2. First-order kinetic plots (a, d; M = monomer), evolution of
molecular weight and molecular-weight distribution (b, e), and GPC
traces (c, f) as a function of conversion for eATRP of OEOMA475
performed in H2O at Eapp = 0.55 V (a–c) or 0.21 V (d–f). Conditions: [monomer]/[RX]/[CuIIL2+] = 200:1:1, [CuIIL2+] = 1 mm, T = 25 8C.
uncontrolled polymerization dominated by termination reactions, such as bimolecular radical–radical coupling as shown
in Figure 2 c (conv. 11 %).
With Eapp = 0.31 V, the current decays slower than in the
case of 0.55 V, but also reaches a constant value except after
a longer duration (see Figure S4 in the Supporting Information). Under these conditions, the rate of polymerization
decreased without significant improvement in polymerization
control (Table 1, entry 2). However, utilizing Eapp = 0.21 V,
the current decays very slowly, approaching a constant value
( 250 mA) within a short period (Figure S4). Note that
Eapp > Eo,
which
implies
an
equilibrium
ratio
[CuIIL2+]/[CuIL+] @ 1. In this case, the polymerization was
observed to be under control ascertained by the linearity of
the first-order kinetic plot (Figure 2 d and Table 1, entry 3).
Nearly quantitative monomer conversion was attained;
furthermore, Mn increases linearly as a function of monomer
conversion and low Mw/Mn values (ca. 1.2) were realized. This
improvement in polymerization control was achieved through
low charge consumption (Q), which serves as evidence of a
drastic decrease in termination events due to an appropriately
supplied balance between [CuIL+] and [CuIIL2+].
To promote the formation of X-CuIIL+, additional experiments were performed in the presence of a large excess of X
(Table 1, entries 4 and 5). As shown, the presence of X
resulted in a significant improvement to the MW distribution
without any loss to the final monomer conversion. Furthermore, the overall rate does not decrease as a result of the
formation of inactive CuIXn species, as previously demonstrated in organic solvents.[11]
Angew. Chem. 2011, 123, 11593 –11596
In an effort to expand the utility of this eATRP process,
we carried out polymerizations under biologically relevant
conditions using a PBS buffer. In this medium Eo’ shifts to
0.326 (see Figure S5 in the Supporting Information), therefore requiring an adjustment of the Eapp to 0.275 V, which
provides an effective Eapp similar to that used in the previous
experiments (Table 1, entries 3 and 4). Although several
interferences perturbing the ATRP equilibrium are plausible
(e.g., formation of insoluble CuII3(PO4)2, stable CuI(H2PO4)2 , and/or CuICl2 [11, 12]), excellent results were
observed both in terms of conversion and Mw/Mn values
(Table 1, entry 5). Indeed, neither displacement of the ligand
nor loss of catalysis was observed by CV analysis.
Last, different targeted DPs were explored (Table 1,
entries 6–8) using 0.1 mm CuIIL2+ (6.4 ppm with respect to
medium) and different concentrations of monomer. All
polymerizations resulted in linear plots of ln([monomer]/[monomer]0) versus time (Figure S6 in the Supporting Information), which together with low Mw/Mn, values,
are indicative of a well-controlled eATRP. Good results were
obtained also in the PBS buffer (Table 1, entry 9).
In conclusion, this work illustrates that eATRP overcomes
many of the drawbacks typically associated with conventional
aqueous ATRP. The CuIIL2+ to CuIL+ ratio, which is crucial to
accomplish a well-controlled polymerization, can be easily
regulated by the appropriate selection of Eapp. The best results
were observed at Eapp > E0CuIIL/CuIL, providing excellent control
over MW and MW distribution, accompanied by a fast
polymerization rate and low charge consumption. Furthermore, eATRP was demonstrated to be tolerant to both
phosphates and halide ions, which additionally served to
provide reduced Mw/Mn values. The presented aqueous
eATRP system may also provide an attractive polymerization
system for the synthesis of biologically relevant materials.
Experimental Section
Electrochemical measurements were performed on a PARC 263A
potentiostat in a thermostatted three-electrode cell under N2
atmosphere, using Pt working electrodes, Pt counter electrode, and
saturated calomel reference electrode (SCE).
Monomer conversions were measured by 1H NMR analysis in
D2O using a Bruker 500 MHz spectrometer. Relative molecular
weights and Mw/Mn values were determined by gel-permeation
chromatography (GPC), using a Waters 2414 RI detector and PSS
columns (Styrogel 105, 103, 102 ) with a calibration based on PMMA
standards (in THF at 35 8C).
Purification of materials as well as procedures and further
experimental details are reported in the Supporting Information.
Received: July 28, 2011
Published online: September 16, 2011
.
Keywords: atom transfer radical polymerization ·
electrochemistry · homogeneous catalysis · polymerization
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